The invention relates generally to surgically implanted devices, and particularly to a surgically implanted self-inflating tissue expander. The invention is capable of expanding skin tissues, mucous membranes, and various other soft tissues, thus allowing subsequent use of these tissues for reconstructive surgery.
It is well known that soft tissue, such as skin, muscle, and mucous membrane, has the ability to expand in order to accommodate the growth of underlying structures. One readily known example is the expansion of abdominal skin and muscle during pregnancy or progressive obesity.
Prior art in the medical world has taken advantage of this observed phenomenon. One such device is the tissue expander of U.S. Pat. No. 4,157,085 issued June 5, 1979 to Austad, the applicant in the present application.
In the above mentioned patent, a sealed envelope, constructed of a material permeable to extracellular body fluids, is surgically implanted adjacent to a site in need of tissue for reconstructive surgery or the like. Silicone rubber is used as an envelope material because of its relative strength and flexibility at the useful thicknesses and its biological compatibility with the human body. A material which establishes a high osmotic pressure is placed within the envelope. This induces the passage of extracellular body fluid into the envelope. Salts, such as sodium chloride, are most useful as the osmotic agent because of the high osmotic pressure they develop and their failure to degrade the envelope material. A quantity of salt is placed within the envelope so as to result in a constant saturated solution. The amount of salt needed will vary depending upon the desired inflation rate and the required final expanded volume.
While the above mentioned invention has shown experimental success, several faults have also been discovered. One is in the use of silicone as the envelope material. In thicknesses adequate for product safety, silicone is relatively impermeable to extracellular water. In order to compensate for the required silicone thickness and allow for timely inflation of the tissue, the implant must contain an inordinately high amount of solute.
A second limitation of the above mentioned patent follows from the first. Rupture of the implanted envelope could result in the dispersion of the highly concentrated solute into the surrounding tissues. If the rupture occurred at or shortly after implantation of the envelope, no adverse effect would occur. However, a rupture occurring after the envelope has approached full inflation would result in severe damage to the overlying tissue. This is believed to occur because of a "jet-stream" dispersion of the saturated salt solution into the overlying tissues, rapidly bathing them with the concentrated solution.
It is an object of the present invention to provide a self-inflating tissue expander which is highly permeable to extracellular water and also provides the required characteristics of elasticity, durability and compatibility. Additionally, the tissue expander must also be relatively impermeable to the solute contained therein. By providing a tissue expander that is highly permeable to extracellular water, the solute concentration level can be reduced to a level that is bio-compatable with the surrounding tissues, even in the unlikely event of a rupture at full inflation.
The present invention incorporates a window portion into the design of a tissue expander. The window portion is highly permeable to extracellular water, while the remaining portion of the implant is generally impermeable to this water. Thus, a significant percentage of the inflation (80-90%) occurs through the window. Both portions, however, must be relatively impermeable to the solute contained within the tissue expander. So constructed, the window-type tissue expander offers great latitude in the choice of implant inflation rates through the manipulation of several variables, including the following: the type of material used for the window, the thickness of the material used for the window, the surface area of the window, and the amount of solute placed within the implant. The window may be incorporated directly into the envelope of the implant, or alternatively, it may be remotely joined to the envelope by a connecter tube.
Two variations on the incorporated-window, self-inflating tissue expander include an implant comprised of two or more separate membrane species and an implant consisting of a single membrane species. The first variation incorporates several types of membranes, each having different characteristics. One or more membranes of a highly permeable material form the window portion of the implant. Inflation of the tissue expander occurs primarily through this region. The second variation of the incorporated-window tissue expander is one composed entirely of a single membrane. The membrane in the window portion of the implant is physically modified to enhance its permeability characteristics without significant loss of other desired characteristics. This window may be a single discrete region of the implant or a number of smaller regions intermittently positioned on the implant.
To facilitate manufacturing and give a Doctor greater control and flexibility with regard to inflation rates, the window portion of the self-inflating tissue expander may be constructed as an interchangeable, remote filling port. The remote window being in communication with the interior of the expandable envelope via a connecting tube.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which the invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.